Small antenna

ABSTRACT

A small antenna for portable radios and the like in which two planar conductive boards are disposed in parallel relationship at a spaced interval which is small when compared with received radio wavelengths. A dielectric frame is disposed between the two planar conductive boards keeping them at the preselected spaced interval. Arbitrary feeding points are selected along the edges of the two conductive boards and at least one conductor is positioned at an arbitrary selected feeding point to short-circuit the two conductive boards. In some cases a plurality of arbitrary feeding points may be selected along the edges of the two conductive boards and a plurality of conductors positioned at the plurality of arbitary selected feeding points which short-circuits the conductive boards. The positions of the plurality of conductors which short-circuit the conductive boards may be automatically changed at will to provide the most desirable condition responsive to the direction of the radio waves.

FIELD OF THE INVENTION

This invention relates to a small antenna for use with a portable smallradio apparatus or the like.

BACKGROUND OF THE INVENTION

In the past it has been well known to provide a loop antenna consistingof a wire rod or loop antenna consisting of a metal strip inside acasing of a radio apparatus or the like.

These small antennas, however, have low radiation resistance, and sohave extremely low efficiency, and the directivity of these loopantennas depends upon the shape of the loop antenna. Still more, thegain of the loop antenna depends upon the area of the loop, which isunder the limitation of the shape or size of the casing, so that it isdifficult to make a radio apparatus more portable and smaller withoutreducing the gain of the antenna.

BRIEF DESCRIPTION OF THE INVENTION

This invention has been made with a view to overcoming the problemsinvolved with loop antennas used for the portable radio apparatuspresently available.

This invention, therefore, has as its principal object the provision ofa novel loop antenna consisting of two rectangular parallel boards withside frames which is small, thin and light in weight.

It is another object of this invention to provide a small antenna whoseefficiency is better than heretofore available.

It is a further object of this invention to provide a small antennawhich can provide a selected directivity characteristic.

According to the invention, the small antenna is comprised of two planarrectangular conductive boards in parallel, dielectric frame providedbetween two feedIng points on the conduction boards along all the fouredges at arbitrary positions along one edge of the two conductive boardsand at least one conductor which short-circuits the two conductiveboards.

The conductive boards and the dielectric frames comprise the casing of aradio apparatus itself as well as the small loop antenna, consequentlythis small antenna has improved efficiency and better directivitycharacteristics than a conventional loop antenna provided inside thecasing of the radio apparatus. Further, by changing the position of theconductor which short-circuits the two conductive boards, it is possibleto change the directivity characteristic. Still the gain of the loopantenna of the invention is higher than that of a conventional antenna.

Other objects advantages and novel features of the invention will becomemore apparent from the following detailed description of the inventionin conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred embodiment of the invention.

FIGS. 2a, 2b, and 2c are graphs showing the direct characteristic of anantenna constructed according to the invention.

FIG. 3 is a chart illustrating spherical coordinates to explain thepolarization directivity characteristic.

FIG. 4 is a graph showing the frequency dependence of radiationefficiency.

FIG. 5a and 5b illustrate the automatic switching of short circuitpositions.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of this invention will now be described withreference to the accompanying drawings.

A first form of a small antenna according to this invention isillustrated in FIG. 1.

The small antenna in this embodiment has two planar rectangularconductive boards I and 2 disposed in parallel at an interval h which isselected to be small when compared with the wavelength of the workingradio wave. Dielectric frame 3 is disposed between and around allopposite edges of conductive boards 1, 2 and the entire conductiveboards 1, 2 and 4 dielectric frame pieces, 3 comprise a casing for aradio apparatus itself.

The casing of this embodiment is a rectangular parallelipiped having alength 1=80 mm, a width w=50 mm, and a heigth (the interval between thetwo boards) h=3.6 mm. Inside this casing are functional circuits for aradio apparatus.

At an arbitrary position in one of the edges of rectangular conductiveboards i and 2 there is a feeding point, which in this embodiment is acorner D-D₁.

At least one conductor 4 positioned between arbitrary positions alongthe edges of the two conductive boards I and 2 short circuit the twoconductive boards I and 2. In this embodiment, conductors 4 may beprovided at one or more selected corners A, A₁ ; B, B₁ ; and C, C₁.

Conductive board I and 2 disposed in parallel act as a loop antenna bythe existence of conductors 4 which shortcircuit the two conductiveboards.

The dimensions 1, w and h and the position of the feeding point (forexample D and D₁ in the first embodiment) and positions of theshort-circuit points are determined by the desired characteristics ofthe antenna and by data experience, with the final decision being madeby experimentation. Interval h between conductive boards i and 2 have aninfluence upon the gain of the antenna, and so long as the value h issufficiently small when compared with the wavelength of a working radiowave, the antenna gain can be made higher with the value h. Accordinglythe dimensions 1, w and h, the position of the feeding point and thepositions for short circuit are decided taking into account the abovementioned matter.

FIG. 2(a) shows the polarized directivity characteristics of theelectrical field along the z axis when one corner angle D-D₁ is afeeding point and a conductor 4 short-circuits corner angles A-A₁. FIG.2(b) shows the characteristics when D-D₁ is a feeding point and theconductor 4 provides a short-circuit between corner angles B-B₁. AndFIG. 2(c) shows the characteristics when D-D₁ is a feeding point and aconductor 4 is provided to short-circuit still another corner angleC-C₁. The symbol φ denotes the angle from the X axis (see FIG. 8). It isapparent from FIG. 2 that if the short-circuit position of conductiveboards i and 2 disposed in parallel is changed at will, it is possibleto change the polarization directivity characteristic of the electricfield and, if the short-circuit position is changed automatically, it ispossible to keep the directivity of the antenna at the most desirablecondition responsive to the direction of a radio wave.

FIG. 4(a)-(c) shows the frequency dependence of radiation efficiency,when two points A, A₁ ; B, B₁ : or C, C₁, respectfully areshort-circuited. A resonant frequency is not changed when A, A₁, B, B₁,C, C₁, or any other points are short-circuited, and radiation efficiencYis independent of the short-circuit position.

FIGS. 5a and 5b illustrate short circuit positions changed automaticallyby switching. In FIGS. 5 a and 5b elements 6, 7, such as pin diodes orcapacitance variable diodes for short circuiting at high frequencies,short circuit corner angles A-A₁, C-C₁ respectively. Radio receiver 8provides a field strength detecting circuit (Receiving Signal StrengthIndicator) which receives a signal from feeding points D-D₁ andgenerates an output signal at 81. Control circuit 5 successively shortcircuits short circuit elements 6, 7 and compares output signal 81 andselects the particular position short circuit elements 6, 7 that providethe maximum signal value at 81.

Actual measurements have confirmed that the present embodiment of theinvention illustrated in FIG. 1 provides high efficiency as good as orbetter than the radio apparatus antennas of prior art.

Having described a specific embodiment having short-circuit points at A,A₁ ; B, B₁, or C, C₁ to make it easy to understand it has been confirmedthat the same effect is, as above achieved, when any other points arethe short-circuit points and even when several points are simultaneousshort-circuit points, or conductor 4 is a metal strip having a certainwidth.

This invention is not to be limited by the embodiment shown in thedrawings and described in the description which is given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

What is claimed is:
 1. A small antenna construction for portable radioor the like comprising;a pair of planar conductive boards disposedparallel to each other; a dielectric insert spacing means positionedbetween and spacing said planar conductive boards at a distance fromeach other that is less than the wavelength of the signals selected tobe received; said planar conductive boards providing a plurality ofarbitrarily selected connecting points on their respective edges forconnecting feed conducting means and short circuits conducting means;means to selectively change said arbitrarily selected connecting pointsin response to the directivity characteristic of the received radiowave; said means to selectively change said arbitrarily selectedconnecting points comprising; a plurality of conductors connected to aplurality of arbitrarily selected connecting points, electronic meansconnected to each of said conductors said electronic means constructedto automatically switch between selected connecting points to change thepolarization characteristic of said small antenna for the bestreception; said electronic means comprising; circuit elements selectedfrom the group of a PIN diode and a variable capacitance diode connectedto each of said plurality of conductors, a radio receiver having a fieldstrength detecting circuit, a control circuit connected between saidradio receiver and said circuit elements for automatically selecting theparticular short circuiting position that provides optimum reception andoutput from said radio receiver, said electronic means including meanscomparing the output signals from said radio receiver and switching tothe short circuiting position that has the best reception; said shortcircuit conducting means comprising at least one conductor connectingsaid planar conductive boards at said arbitrarily selected connectingpoints on their respective edges; whereby the polarized directivitycharacteristics of said antenna for said radio or the like may bechanged by changing the position of said at least one conductor.
 2. Thesmall antenna according to claim 1 wherein said dielectric insertspacing means is disposed around the edges of said planar conductiveboards whereby said dielectric insert and said conductive boards form anenclosure for a radio circuit.
 3. The small antenna according to claim 1wherein said planar conductive boards are approximately equal in size.4. The small antenna according to claim 3, wherein said planarconductive boards are rectangular; said arbitrarily selected connectingpoints being selected at opposing respective corner points of saidplanar conductive boards.
 5. The small antenna according to claim 4wherein said dielectric insert spacing means is disposed around theedges of said planar conductive boards whereby said dielectric insertand said conductive boards form an enclosure for a radio circuit.